1. Field of the Invention
This invention relates to charge transfer devices, particularly but not exclusively for use as filters, analog to digital converters, or digital to analog converters.
2. Description of the Prior Art
From "Digest of Technical Papers of the Technology and Applications of Charge-Coupled Devices", University of Edinburgh, September 1976, page 308, FIG. 7, a charge transfer device is known comprising a semiconductor layer of one conductivity type, write means for permitting information in the form of packets of charge to be locally introduced into the semiconductor layer, read means for permitting said information to be read at another location in the layer and control electrodes at least on one side of the layer for permitting electric fields to be capacitively generated in the semiconductor layer with the aid of multiphase clock signals, by means of which fields the charge packets can be transferred to the read means along a channel in the layer in a direction parallel to the layer.
In this known charge transfer device the write means are constituted by two input diffusions, three control electrodes and one isolation diffusion. The isolation diffusion is disposed in the direction of transfer of the charge packets and divides the transfer channel at the input of the charge transfer device into two sections. Each transfer section includes an input diffusion. These two diffusions are jointly connected to a source of reference voltage. Above each of the two transfer sections there is arranged a first control electrode. Different clock signals are applied to the two first control electrodes. Subsequently, a common second control electrode is arranged over the two transfer sections, to which the input signal to be processed is applied. Subsequently, after the write means a third and a fourth control electrode are arranged adjacent each other above the one transfer section, while opposite these two control electrodes a fifth control electrode is arranged above the other channel. The surface area of this fifth control electrode is substantially equal to the sum of the surface areas of the third and the fourth control electrodes. After the last-mentioned control electrodes the isolation diffusion ceases and the transfer channel is no longer split into two sections. Above the further channel there are arranged control electrodes, which are split or non-split, the first control electrode constituting the summing electrode.
The fact that above the one channel section there are arranged a third and a fourth control electrode, while above the corresponding other channel section there is arranged only one control electrode, results in the signal being transferred in two steps in the one channel section, while the signal is conveyed in one step in the other channel section. This means that the signal in the one channel section is delayed relative to the signal in the other channel section. After the control electrode the isolation diffusion ceases and so also does the channel division. Underneath the summing electrode the signals from the two channel sections are added to each other. For a correct charge transfer underneath the summing electrode it is necessary that the clock phase of the clock signals which are applied to the control electrodes which precede the summing electrode in the two channel sections are identical. As the number of steps of the charge transfer for the summing electrode in the two channel sections is different, it is necessary that the sampling instants at the input of the two channel sections are selected differently. In the present example this is realized by the application of clock signals which are shifted 180.degree. in phase relative to each other. Thus, underneath the summing electrode the sum obtained is of two signals, one of which is delayed relative to the other. The circuit operates in fact as a forward filter, as for example shown in FIG. 1 on page 306 of the cited literature. In the aforementioned example the transfer channel is divided into two sections, the signal in the one section being transferred in one more step. If a sharper input filter is required, the transfer channel is split into several sub-channels at the beginning of the charge transfer device, which sub-channels all have a different delay relative to each other for the signal in the relevant sub-channel. Examples of this are given in FIGS. 8 and 9 on page 308 of the cited literature.
In the cited reference forward filtration is always employed at the input of the charge transfer device. However, in principle, it is also possible to apply the filtration not at the input but further on in the charge transfer device. However, the only condition is then that the delays in the channel sections amount to an integral number of clock periods, because otherwise the signals in the various channel sections do not arrive at the summing electrode with the correct clock phase.
A disadvantage of the method of signal processing described is that the described method only allows time-invariant processing of the input signal and furthermore that in the case of operation as a filter the transfer function of the filter is non-variable. This is because this function is entirely determined by the selected geometry of the section of the charge transfer device belonging to the filter. It is as though the transfer function is "built in" by the choice of the said geometry.
A further drawback is that the areas of the control electrode are selected differently so as to realize the required filter function. As a result of this, the signal transfer underneath the widened control electrodes takes place more slowly than underneath the non-widened control electrodes, namely more slowly according as the relevant control electrode is further widened. This means that the maximum attainable transfer speed of the charge transfer device is drastically reduced and decreases further when wider control electrodes are used.
Another disadvantage is that the large variety in control electrode areas complicates manufacture of the known charge transfer device. Moreover, the application of contact conductors between the various control electrodes and the associated clock conductors becomes more difficult as the areas of the respective control electrodes differ more from each other.